74 HARKER: SOME NORTH-COUNTRY QUARTZITES. 
It may be noticed that when the growth of the quartz-grains as 
described above, has not proceeded so far as to completely fill the 
interstices, the enlarged grains sometimes show the external form of 
perfect crystals. This was noticed long ago by Dr. Sorby, in the 
Penrith Sandstone, and beautiful specimens may be obtained on 
Penrith Beacon. In sandstones which contain fragments of felspar, 
these, like the quartz, sometimes show a secondary enlargement, 
but the phenomenon is much less common. It is seen occasionally 
in the Roman Fell quartzite. 
The quartzites formed by thermal metamorphism, where sand- 
stones come into contact with some large body of igneous rock, have 
often a very similar general appearance to those referred to above, 
but the different origin is clearly shown in thin slices. Here there is 
no distinction of original nucleus and secondary fringe; the outlines 
of the original grains are completely obliterated, and the slice is seen 
to be an irregular mosaic of perfectly clear grains, which interlock 
sometimes in quite an intricate fashion. It is evident that the 
whole rock has been recrystallised in ice One good example is 
furnished by a rock collected in Teesdale, where a fine-grained 
light-coloured sandstone of Carboniferous age comes into contact 
with the Great Whin Sill. The locality is on the Durham side of 
the river, about a quarter of a mile above High Force. The 
recrystallised grains interlock quite irregularly, or, in some places, 
are separated by little patches of an ill-defined, scaly, chloritic 
grains are not exclusively of quartz. Here and there the lamellar 
twinning of a triclinic felspar is to be seen. New-formed felspar of 
this kind is perfectly limpid in appearance, and in some quartzites, 
felspar and quartz are to be distinguished only by special optical 
tests, the grains of the former mineral being not always twinned. 
Some of the crystalline grains in the Teesdale rock contain minute 
cavities occupied by glass, a not infrequent feature of thermal meta- 
morphism. 
In impure sandstones and grits, metamorphism may give rise to 
other minerals than the simply recrystallized quartz and felspar. At 
Packhouse Hill, near Shap Wells, the grits of the Coniston Flags 
come within about 600 yards of the Shap granite, and are highly 
metamorphosed. A thin slice shows, under the microscope, the 
usual mosaic of clear grains, mostly quartz but with some felspar 
(apparently orthoclase) ; but, in addition, there are numerous little 
rounded granules, colourless or faint yellow, and very brightly 
polarising. These are referred to a pyroxene, rich in lime, a 
Naturalist, — 
